Using the predicate language with the redirect() handler does not provide a way to control whether a 302 or 301 redirect is issued.  The handler is hard coded to use a 302. 

path('/a') -> redirect('/b')

The response-code() handler can presumably be used in concert with the redirect handler, but it would be much simpler to allow something like this instead: 

path('/a') -> redirect(value='/b',status=301)